I'm having a problem with a Linux system and I have found sysstat and sar to report huge peaks of disk I/O, average service time as well as average wait time.

How could I determine which process is causing these peaks the next time it happen?

Is it possible to do with sar? Can I find this info from the already recorded sar files?

Output of sar -d, system stall happened around 12.58-13.01pm.

12:40:01          DEV       tps  rd_sec/s  wr_sec/s  avgrq-sz  avgqu-sz     await     svctm     %util
12:40:01       dev8-0     11.57      0.11    710.08     61.36      0.01      0.97      0.37      0.43
12:45:01       dev8-0     13.36      0.00    972.93     72.82      0.01      1.00      0.32      0.43
12:50:01       dev8-0     13.55      0.03    616.56     45.49      0.01      0.70      0.35      0.47
12:55:01       dev8-0     13.99      0.08    917.00     65.55      0.01      0.86      0.37      0.52
13:01:02       dev8-0      6.28      0.00    400.53     63.81      0.89    141.87    141.12     88.59
13:05:01       dev8-0     22.75      0.03    932.13     40.97      0.01      0.65      0.27      0.62
13:10:01       dev8-0     13.11      0.00    634.55     48.42      0.01      0.71      0.38      0.50

I also have this follow-up question to another thread I started yesterday:


6 Answers 6


If you are lucky enough to catch the next peak utilization period, you can study per-process I/O stats interactively, using iotop.

  • Running iotop in batch mode could be a very good complement/replacement for the "ps -eo" solution above. Thanks! Aug 12, 2010 at 11:43
  • 2
    Awesome, "iotop -n 1 -b -o" provides exactly the output i need. Thanks! Aug 12, 2010 at 11:59
  • 1
    looks like this requires root access to the system to run May 1, 2018 at 21:25
  • If the disk is heavily loaded, then iotop freezes and is useless.
    – Nathan
    Mar 4, 2021 at 23:51

You can use pidstat to print cumulative io statistics per process every 20 seconds with this command:

# pidstat -dl 20

Each row will have follwing columns:

  • PID - process ID
  • kB_rd/s - Number of kilobytes the task has caused to be read from disk per second.
  • kB_wr/s - Number of kilobytes the task has caused, or shall cause to be written to disk per second.
  • kB_ccwr/s - Number of kilobytes whose writing to disk has been cancelled by the task. This may occur when the task truncates some dirty pagecache. In this case, some IO which another task has been accounted for will not be happening.
  • Command - The command name of the task.

Output looks like this:

05:57:12 PM       PID   kB_rd/s   kB_wr/s kB_ccwr/s  Command
05:57:32 PM       202      0.00      2.40      0.00  jbd2/sda1-8
05:57:32 PM      3000      0.00      0.20      0.00  kdeinit4: plasma-desktop [kdeinit]              

05:57:32 PM       PID   kB_rd/s   kB_wr/s kB_ccwr/s  Command
05:57:52 PM       202      0.00      0.80      0.00  jbd2/sda1-8
05:57:52 PM       411      0.00      1.20      0.00  jbd2/sda3-8
05:57:52 PM      2791      0.00     37.80      1.00  kdeinit4: kdeinit4 Running...                   
05:57:52 PM      5156      0.00      0.80      0.00  /usr/lib64/chromium/chromium --password-store=kwallet --enable-threaded-compositing 
05:57:52 PM      8651     98.20      0.00      0.00  bash 

05:57:52 PM       PID   kB_rd/s   kB_wr/s kB_ccwr/s  Command
05:58:12 PM       202      0.00      0.20      0.00  jbd2/sda1-8
05:58:12 PM      3000      0.00      0.80      0.00  kdeinit4: plasma-desktop [kdeinit]              
  • 1
    Great answer! Please note that pidstat is usually not installed by default - on Ubuntu you'd need to install sysstat to get it. To look for IO of specific processes, use either -G <process_name> or -p <pid>. Also, if you want a snapshot (not continually updating), add 1 after the command in the answer (man pidstat for more details), for example: pidstat -G suspect_proc -dl 20 1
    – Guss
    Mar 19, 2020 at 22:46
  • If the disk is heavily loaded, then pidstat with some parameters freezes and is useless.
    – Nathan
    Mar 5, 2021 at 0:04
  • Note: even though pidstat doesn't require sudo, it won't show the read/write speed unless it runs with root rights
    – Finesse
    May 16 at 0:21

Nothing beats ongoing monitoring, you simply cannot get time-sensitive data back after the event...

There are a couple of things you might be able to check to implicate or eliminate however — /proc is your friend.

sort -n -k 10 /proc/diskstats
sort -n -k 11 /proc/diskstats

Fields 10, 11 are accumulated written sectors, and accumulated time (ms) writing. This will show your hot file-system partitions.

cut -d" " -f 1,2,42 /proc/[0-9]*/stat | sort -n -k +3

Those fields are PID, command and cumulative IO-wait ticks. This will show your hot processes, though only if they are still running. (You probably want to ignore your filesystem journalling threads.)

The usefulness of the above depends on uptime, the nature of your long running processes, and how your file systems are used.

Caveats: does not apply to pre-2.6 kernels, check your documentation if unsure.

(Now go and do your future-self a favour, install Munin/Nagios/Cacti/whatever ;-)


Use atop. (http://www.atoptool.nl/)

Write the data to a compressed file that atop can read later in an interactive style. Take a reading (delta) every 10 seconds. do it 1080 times (3 hours; so if you forget about it the output file won't run you out of disk):

$ atop -a -w historical_everything.atop 10 1080 &

After bad thing happens again:

(even if it is still running in the background, it just appends every 10 seconds)

% atop -r historical_everything.atop

Since you said IO, I would hit 3 keys: tdD

t - move forward to the next data gathering (10 seconds)
d - show the disk io oriented information per process
D - sort the processes based on disk activity
T - go backwards 1 data point (10 seconds probably)
h - bring up help
b - jump to a time (nearest prior datapoint) - e.g. b12:00 - only jumps forward
1 - display per second instead of delta since last datapiont in the upper half of the display
  • 1
    atop does not freeze if the disk is heavily loaded
    – Nathan
    Mar 5, 2021 at 0:09

Use btrace. It's easy to use, for example btrace /dev/sda. If the command is not available, it is probably available in package blktrace.

EDIT: Since the debugfs is not enabled in the kernel, you might try date >>/tmp/wtf && ps -eo "cmd,pid,min_flt,maj_flt" >>/tmp/wtf or similar. Logging page faults is not of course at all the same than using btrace, but if you are lucky, it MAY give you some hint about the most disk hungry processes. I just tried that one on of my most I/O intensive servers and list included the processes I know are consuming lots of I/O.

  • Hello Janne, the kernel is unfortunately not compiled with debug file system, and its a live system so I am unable to recompile the kernel. Is there any other way to do this without recompiling? Aug 12, 2010 at 8:13
  • OK, I edited my reply a bit :) Aug 12, 2010 at 8:27
  • Great, now we're getting somewhere! Im thinking about putting this into a cronjob and execute it concurrently with the sar cron job. Then, next time the server stalls I should be able to compare the rate of the page faults to see which process/processes has an increased rate of page faults. I guess I could be unlucky and see a raise in disk io for all processes during the stall, but its definately worth a good try. Thanks Janne! (i would vote up on your answere if i could :S) Aug 12, 2010 at 9:02
  • You're welcome. Let me know how it went, this was just a creative problem solving attempt from me. :-) Aug 12, 2010 at 9:55
  • The iotop output is easier to under interpret, so ill accept that solution. Ill be back to vote up on your answere as soon as I have earned rep enough to do so. Thanks for your support! Aug 12, 2010 at 12:00

Disk utilization by each process:

$ glances # (with htop the best tool to get idea what is going on. Hit right arrow keys for process sorting by disk utilization)

$ sudo iotop -ao # (-a accumulated; -o show only processes with activity)

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